This invention relates to a process for the synthesis of thionocarbamates.
Thionocarbamates are well known compounds used as reagents in ore floatation. These compounds are also referred to as thiourethanes and are represented by the general formula: ##STR1## where R and R.sup.1 are alkyl radicals.
The synthesis of thionocarbamates is also well known in the art. At the present time, there are two general methods for this synthetic process. The first method is a two-step reaction which involves reacting an alkali metal xanthate with an alkyl halide to form a diakyl xanthate. The xanthate is then reacted with an amine to form a thionocarbamate and a mercaptan. The second method is a one-step catalytic reaction which involves reacting a suitable xanthate and an amine in the presence of a suitable catalyst.
The problems and drawbacks of the first method are of common knowledge to those skilled in the art. Bolth, et al., (U.S. Pat. Nos. 3,975,264 and 3,907,854) background of the invention sections, which are hereby incorporated by reference, contain an excellent discussion of said problems and drawbacks. Furthermore, Bolth, et al., U.S. Pat. No. 3,907,854 represent a commercially accepted manufacturing process. In Bolth, et al., dissolved nickel salts (nickel sulfate) and palladium salts are used as catalysts. Palladium salts, for some inexplicable reasons seem to work better and are the catalyst of choice.
Although the Bolth, et al., process constitutes a significant improvement over the fomer two-step reaction process, it too, has its own drawbacks especially when nickel salts are used as catalyst. First, the nickel sulfate salt, because of its high solubility, completely goes into the aqueous phase. This makes its recovery and reuse economically unfeasible. Second, the process requires using relatively large quantities of nickel. Because nickel is costly, this further escalates the overall cost of the process. Third, the difficulty of recovering the nickel leads to potential environmental hazards: nickel, a toxic element, must now be disposed of while in the form of a salt dissolved in water.
These problems and the increased demand to synthesize thionocarbamates for the mining industry, provide a constant incentive for the development of alternative and better methods.